OF THE BLOOD.
Having described the course which the blood pursues along the different parts of the sanguiferous system, we must now proceed to consider the nature of this fluid. Blood may be regarded both in a chemical and a physio logical point of view ; either as it is a substance possessed of certain chemical properties, or as an agent which ma terially influences the vital powers of the system, and is intimately connected with the performance of its most important functions. The first of these subjects we shall pass over in a very cursory manner, as it has already been treated of under the article CHEMISTRY; while we shall consider more in detail those properties of the blood which are especially connected with it, as forming a con stituent of the living animal body.
Blood, when first drawn from the vessels, is a tenacious fluid of a thick consistence; in man, and the more per fect animals, of a red colour, of homogeneous consistence, of a specific gravity greater than water, and of the tem perature of the animal from which it is taken ; this, in the human species, is about 98°. Very soon after it leaves the vessels, it separates into two distinct parts ; a solid mass is formed of a red colour, called the clot or crassa mentum, which floats in a yellowish fluid, 'named the se rum ; each of these bodies, however, are composed of va rious other substances : these we shall describe in suc cession. The crassamentum generally appears under the form of a soft solid, of such a consistence as to bear cut ting with a knife. When formed under particular cir cumstances, especially if it be gently agitated during the time of its coagulation, it exhibits a fibrous appearance, and, by proper management, nearly the whole of it may be converted into an irregular net-work of a dense fibrous substance; in this case it is generally deprived of its red colour. The red colour may also be removed from it by long-continued washing ; and, in this case, the same fibrous texture is also developed. The substance thus obtained, consisting of the coagulum deprived of its red colour, was formerly called coagulable lymph or gluten ; but lately it has obtained the more appropriate name of fibrin. The washing removes from the coagulum a quantity of serum which was entangled, as it were, between its fibres, and also the substance to which it owes its red colour ; the fibrin is then left nearly white, of a firm consistence, and possessed of a certain degree of toughness and elasticity. It is upon
this part that the separation of the clot depends ; a pro cess which has been termed the spontaneous coagulation of the blood.
So singular a change could not but excite great atten tion among physiologists ; and very numerous experi ments have been made, both to explain its occurrence, and to observe what causes tended to promote or to retard it. Two of the most remarkable circumstances which might be supposed to operate as constituting the chief difference between the blood while in the vessels, and after it is dis charged from them, are, rest and exposure to air. If blood, Ivlien newly drawn from the vessels, be briskly agitated, the process of coagulation is entirely prevented from tak ing place; either in consequence of a more complete union of its constituents, which prevents their subsequent separation, or from the fibrin losing the property by which its parts are attracted to each other. Hewson performed a number of experiments on the effect of air upon the spontaneous coagulation of the blood ; and, although they are, perhaps, not very decisive, nor always uniform, yet the general conclusion is, that the contact of air promotes coagulation. Hunter opposed the opinion of Hewson ; but his facts go no farther than to show, that air is not es sential to the process. This point is indeed proved by the consideration, that the blood occasionally coagulates within the vessels, where it must be completely excluded from the air ; and, indeed, this change has been found to exist, to a certain degree, during life, as seems to be the case in aneurisms, where a part of the blood is removed from the direct impulse of the heart's action. Many wonderful stories that are on record, of worms being found in the heart, the brain, or other internal organs, are to be explain ed upon the idea, that the spectators were imposed upon by portions of coagulated fibrin, which had assumed the form of long strings.